Method of manufacturing central electrode for spark plug

ABSTRACT

A method of manufacturing a central electrode for a spark plug in which an elongated rod consisting of a core of highly heat conductive metal and an outer sheath of corrosion and heat resisting metal is cut into predetermined lengths to obtain unit rods, and part of the core is removed from the firing end of the unit rod to leave a recess in such end. Subsequently, the outer sheath portion surrounding the recess is bent inwardly to close completely the firing end without using any brazing material.

United States Patent [191 Yamaguchi et al.

[4 June 25, 1974 METHOD OF MANUFACTURING CENTRAL ELECTRODE FOR SPARK PLUG [75] inventors: Terumoto Yamaguchi, Anjo;

Kanemithi Izumi, Oobu, both of Japan [73] Assignee: Nippondenso Co., Ltd., Kariya-shi Aichi-ken, Japan [22] Filed: Mar. 16, 1973 [21] Appl. No.: 341,852

[30] Foreign Application Priority Data 3,119,944 l/l964 Lentz et al 313/141 Primary Examiner-Roy Lake Assistant Examiner-J. W. Davie 1 Attorney, Agent, or Firm-Cushman, Darby &

Cushman [57] ABSTRACT A method of manufacturing a central electrode for a spark plug in which an elongated rod consisting of a core of highly heat conductive metal and an outer sheath of corrosion and heat resisting metal is cut into predetermined lengths to obtain unit rods, and part of the core is removed from the firing end of the unit rod to leave a recess in such end. Subsequently, the outer sheath portion surrounding the recess is bent inwardly to close completely the firing end without using any brazing material.

6 Claims, 8 Drawing Figures METHOD OF MANUFACTURING CENTRAL ELECTRODE FOR SPARK PLUG BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method of manufacturing a spark plug and more particularly to a method of manufacturing a spark plug center electrode including a core of material such as copper having a high heat conductivity and an outer sheath of material such as a nickel alloy having a high resistance to corrosion and heat.

2. Description of the Prior Art Various methods of manufacturing spark plug center electrodes have been proposed heretofore and are disclosed in, for example, Japanese Patent Publications Nos. 10309/1961, 6874/1968 and 38847/1970. Japanese Patent Publication No. 10309/1961 discloses a method of manufacturing a center electrode in which a cold pressing punch is used to apply a force to two disc-shaped pieces of corrosion resisting metal and highly heat conductive metal. However, this method has been defective in that an expensive cold forging press is required, a thin thickness cannot be obtained in the corrosion resisting metal layer portion at the firing end, and the outer sheath of the corrosion resisting metal is not uniform. According to Japanese Patent Publication No. 6874/1968, a suitably internally stepped outer sheath of heat resisting metal having a closed bottom is employed and a core of highly heat conductive metal is fitted into the opening of the outer sheath in such a state that the head portion of the core projects outwardly from the open end of the outer sheath as required. According to Japanese Patent Publication No. 38847/1970, a piece of corrosion resisting metal is subjected to cold working to be finished to substantially the desired size of the outer sheath, and a core of highly heat conductive metal having been cut into a desired length or subjected to cold extrusion after cutting is press fitted into the opening of the outer sheath. However, both these two methods have been defective in that a plurality of expensive cold forging steps are required for forming the piece of corrosion resisting metal into the cup-like shape for assembling same with the core of highly heat conductive metal resulting in an increase in the number of manufacturing steps. These methods have been further defective in that heat must be especially applied to cause diffusion of the metals toward each other in order to ensure a good bond between the outer sheath and the core.

US. Pat. No. 3,1 19,944 discloses another method of manufacturing a spark plug center electrode. This method comprises providing a rod including a core of highly heat conductive metal and a sheath therearound of metal having high resistance corrosion, removing part of the core from one end of the rod to leave a recess therein, placing powdery brazing material in said recess, heating said rod and said powder to a temperature at which said powder will melt and at least partially fill said recess, and cooling the combination. This method has however been defective in that a powdery brazing material is especially required.

SUMMARY OF THE INVENTION With a view to obviate these prior art defects, the present invention contemplates the provision of an improved method of manufacturing a center electrode for a spark plug comprising the steps of cutting an elongated rod consisting of a core of highly heat conductive material and an outer sheath of corrosion and heat resisting material into unit rods of predetermined length, removing part of said core of highly heat conductive material from the firing end of said unit rod to leave a recess in the firing end, and applying a force to the firing end for bending the portion of said outer sheath of corrosion resisting material surrounding said recess inwardly so as to bring said outer sheath portion into intimate contact with the corresponding portion of said core of highly heat conductive material thereby closing said firing end.

It is an object of the present invention to provide an improved method of manufacturing a center electrode for a spark plug which eliminates the necessity for expensive cold forging steps and can reduce the thickness of the outer sheath of corrosion and heat resisting material forming the recess in the firing end before such end is finally closed, thereby ensuring satisfactory dissipation of heat.

Another object of the present invention is to provide a method of manufacturing a spark plug center electrode which does not require a third component such as a brazing material and has asatisfactory resistance to heat.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partly cut-away plan view of a rod employed in a method according to the present invention.

FIG. 2 is a longitudinal sectional view of a unit rod obtained by cutting the elongated rod shown in FIG. 1 into predetermined lengths.

FIGS. 3 and 4 are longitudinal sectional views showing the state in which part of the core of highly heat conductive material is removed from the firing end of the unit rod shown in FIG. 2.

FIG. 5 is a longitudinal sectional view showing a preparatory step taken before closing the firing end of the unit rod shown in FIG. 3 or 4.

FIG. 6 is a longitudinal sectional view showing the state in which the firing end of the unit rod shown in FIG. 5 is completely closed.

FIG. 7 is a partly sectional front elevation showing a device for closing the firing end of the unit rod shown in FIG. 5.

FIG. 8 is a longitudinal sectional view showing application of welding to the closed portion of the firing end of the unit rod shown in FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIGS. 1 and 2 of the drawings showing a first step of a method of manufacturing a spark plug center electrode according to the present invention an elongated rod la consisting of a core 2 of highly heat conductive material such as copper and an outer sheath 1 of corrosion and heat resisting material such as an alloy containing nickel and chromium shown in FIG. I is cut by means such as a press or lathe to obtain a unit rod as shown in FIG. 2. In a second step, part of the core 2 of highly heat conductive material is removed as by cutting from the firing end 3a of the unit rod obtained by the first step to form a recess 3 surrounded by a portion of the outer sheath 1 of corrosion and heat resisting material as shown in FIG. 3. In a modification of the second step, this recess 3 is formed in the firing end 3a by imparting a force solely to the core 2 of highly heat conductive material while firmly holding the outer sheath 1 of corrosion and heat resisting material thereby displacing the position of the core 2 relative to the outer sheath 1 as shown in FIG. 4. Such recess 3 can also be formed in the firing end 3a by applying backward extrusion. The portion 7 of the core 2 of highly heat conductive material projecting from the end of the outer sheath 1 remote from the firing end 3a may be removed by cutting if such projection provides a hindrance to the assembling of the spark plug structure.

A third step includes a preparatory step for ensuring complete closure of the firing end 3a. In this preparatory step, the portion lb of the outer sheath 1 of corrosion and heat resisting material surrounding the recess. 3 is preferably bent inwardly at an angle 2 of 45 to 80. The diameter d of the opening 4 formed thereby is preferably as small as possible. For example, this diameter d is less than 0.3 mm. A semi-finished product 8 thus obtained is placed in an opening of a die as shown in FIG. 7 and a force is imparted thereto by an upper and a lower pressing member 9 and 11 so as to bring the outer sheath portion 1b defining the recess 3 into intimate contact with the corresponding portion of the core 2 of highly heat conductive material. In this case, a minute cavity 5 may be produced as shown in FIG. 6 due to slight dimensional errors in the semi-finished product 8. In order to avoid occurrence of such minute cavity 5, the upper and lower pressing members 9 and 11 may be arranged to impart a predetermined force to the ends of the semifinished product 8 to obtain better results than when the lower dead center the upper pressing member 9 is mechanically determined.

When the presence of a minutest cavity is not allowed at the firing end 3a, a welding method such as a plasma arc welding method or electron beam welding method may be employed to concentrate welding energy to the minute cavity 5 shown in FIG. 6 thereby causing fusion of the metal in this area. FIG. 8 shows a center eletrode thus obtained. The fused zone 6 shown in FIG. 8 need not extend to the core 2 of highly heat conductive material and the corrosion and heat resisting metal in this area may be such that it can deal with consumption and corrosion due to spark discharge.

It will be understood from the foregoing description that the present invention provides a method of manufacturing a center electrode for a spark plug comprising the steps of cutting an elongated rod consisting of a core of highly heat conductive material and an outer sheath of corrosion and heat resisting material into unit rods of predetermined length, removing part of said core of highly heat conductive material from the firing end of said unit rod to leave a recess in the firing end, and applying a force to the firing end for bending the portion of said outer sheath of corrosion and heat resisting material surrounding said recess inwardly so as to bring said outer sheath portion into intimate contact with the corresponding portion of said core of highly heat conductive material thereby closing said firing end. The present invention is therefore remarkably advantageous in that expensive working such as cold forging is unnecessary and the center electrode can be manufactured by very simple steps.

Further, the present invention which employs the rod of the structure above described as a blank is remarkably advantageous in that the thickness of the outer sheath of corrosion and heat resisting material can be selected as desired and is uniform, and the outer sheath portion at the firing end has a smaller thickness than heretofore so that dissipation of heat required for the center electrode can be remarkably improved. Furthermore, due to the fact that any brazing material is not used for closing the firing end and the outer sheath portion of corrosion and heat resisting material is merely bent inwardly to close the firing end, a third component such as a brazing material is not required, and the center electrode consisting of the core of highly heat conductive material covered solely with the outer sheath of corrosion and heat resisting material is highly resistant to heat even when it is exposed to a high temperature atmosphere. Moreover, the present invention which does not employ a brazing material is remarkably advantageous in that there is utterly no fear of dropping of the brazing material into the engine cylinders.

We claim:

1. A method of manufacturing a center electrode for a spark plug comprising the steps of cutting an elongated rod consisting of a core of highly heat conductive material and an outer sheath of corrosion and heat resisting material into unit rods of predetermined length, removing part of said core of highly heat conductive material from one end, termed the firing end, of said unit rod to leave a recess in the firing end, and applying a force to the firing end for bending the portion of said outer sheath of corrosion and heat resisting material surrounding said recess inwardly so as to bring said outer sheath portion into intimate contact with the corresponding portion of said core of highly heat conductive material thereby closing said firing end.

2. A method as claimed in claim 1, wherein said force applying step comprises the step of bending the outer sheath portion of corrosion and heat resisting material inwardly at an angle of 45 to to constrict the diameter of the opening to less than 0.3 mm, and the step of inserting a semi-finished product thus obtained in a die and imparting a predetermined force thereto by suitable force imparting means thereby bringing the outer sheath portion of corrosion and heat resisting material into intimate contact with the corresponding portion of said core of highly heat conductive material for closing the firing end.

3. A method as claimed in claim 2, wherein said highly heat conductive material is copper and said corrosion and heat resisting material is an alloy containing nickel and chromium.

4. A method as claimed in claim I, wherein said force applying step comprises further the step of fusing the corrosion and heat resisting material portion around a minute cavity which may be produced at the firing end so as to completely fill the cavity with the material without using any other material.

5. A method as claimed in claim 2, wherein said force applying step comprises further the step of fusing the corrosion and heat resisting material portion around a minute cavity which may be produced at the firing end so as to completely fill the cavity with the material without using any other material.

6. A method as claimed in claim 5, wherein said highly heat conductive material is copper and said corrosion and heat resisting material is an alloy containing nickel and chromium. 

1. A method of manufacturing a center electrode for a spark plug comprising the steps of cutting an elongated rod consisting of a core of highly heat conductive material and an outer sheath of corrosion and heat resisting material into unit rods of predetermined length, removing part of said core of highly heat conductive material from one end, termed the firing end, of said unit rod to leave a recess in the firing end, and applying a force to the firing end for bending the portion of said outer sheath of corrosion and heat resisting material surrounding said recess inwardly so as to bring said outer sheath portion into intimate contact with the corresponding portion of said core of highly heat conductive material thereby closing said firing end.
 2. A method as claimed in claim 1, wherein said force applying step comprises the step of bending the outer sheath portion of corrosion and heat resisting material inwardly at an angle of 45* to 80* to constrict the diameter of the opening to less than 0.3 mm, and the step of inserting a semi-finished product thus obtained in a die and imparting a predetermined force thereto by suitable force imparting means thereby bringing the outer sheath portion of corrosion and heat resisting material into intimate contact with the corresponding portion of said core of highly heat conductive material for closing the firing end.
 3. A method as claimed in claim 2, wherein said highly heat conductive material is copper and said corrosion and heat resisting material is an alloy containing nickel and chromium.
 4. A method as claimed in claim 1, wherein said force applying step comprises further the step of fusing the corrosion and heat resisting material portion around a minute cavity which may be produced at the firing end so as to completely fill the cavity with the material without using any other material.
 5. A method as claimed in claim 2, wherein said force applying step comprises further the step of fusing the corrosion and heat resisting material portion around a minute cavity which may be produced at the firing end so as to completely fill the cavity with the material without using any other material.
 6. A method as claimed in claim 5, wherein said highly heat conductive material is copper and said corrosion and heat resisting material is an alloy containing nickel and chromium. 